New observational studies show that increasing air dryness does not decrease photosynthesis in certain very humid regions of the Amazon rainforest, which contradicts Earth system models, which show the opposite
COLUMBIA UNIVERSITY SCHOOL OF ENGINEERING AND APPLIED SCIENCE
IMAGE: The photo was taken from the top of the K34 FLUX TOWER site, which is 60 km north of Manus, Brazil. Show more CREDIT: XI YANG / UNIVERSITY OF VIRGINIA
New York, NY – November 20, 2020 – Forests can help curb climate change by absorbing carbon dioxide during photosynthesis and storing it in their biomass (tree trunks, roots, etc.). In fact, forests currently absorb around 25 to 30% of our human-made carbon dioxide (CO2) emissions. Certain rainforest regions like the Amazon store more carbon in their biomass than any other ecosystem or forest. However, when the forests are under water stress (not enough water in the soil and / or the air is extremely dry), the forests slow down or stop photosynthesis. This leaves more CO2 in the atmosphere and can also lead to tree mortality.
The current earth system models that are used for climate predictions show that the Amazon rainforest is very sensitive to water stress. Since the air is expected to become warmer and drier in the future with climate change, which is reflected in increased water stress, this could have a major impact not only on the survival of the forest, but also on its storage of CO2. If the forest cannot survive in its current capacity, climate change could accelerate significantly.
Columbia Engineering researchers decided to investigate whether this was true and whether these forests were really as sensitive to water stress as the models. In a study published today in Science Advances, they report their discovery that these models have largely overestimated water stress in tropical forests.
The team found that while models show that an increase in air dryness greatly reduces photosynthesis rates in certain regions of the Amazon rainforest, the results of the observational data show the opposite: in certain very humid regions, forests instead actually increase photosynthesis rates in response on drier air.
"To the best of our knowledge, this is the first basin-wide study to show how photosynthesis actually increases in some very humid regions of the Amazon rainforest with limited water stress," said Pierre Gentine. Associate Professor of Earth and Environmental Technology and Earth and Environmental Sciences and member of the Earth Institute. “In addition to the radiation, this increase is also related to the dryness of the atmosphere and can largely be explained by changes in the photosynthesis capacity of the canopy. When the trees are stressed, they produce more efficient leaves which can more than offset the water stress. "
Gentine and his former PhD student Julia Green used data from the CMIP5 (Coupled Model Intercomparison Project 5) model of the Intergovernmental Panel on Climate Change and combined them with machine learning techniques to determine the modeled sensitivity of photosynthesis in both tropical regions of America Soil moisture as well as air dryness. They then performed a similar analysis, this time using observation remote sensing data from satellites instead of the model data to see how the observation sensitivity was compared. To relate their results to smaller processes that they could explain, the team then used Flux Tower data to understand their canopy and leaf-level results.
Previous studies have shown that the green in the Amazon Basin increases at the end of the dry season, when both the ground and air are drier, and some have linked this to an increase in photosynthesis. "Before our study, however, it was unclear whether these results had an impact on a larger region, and apart from light they had never been associated with dry air," said Green, who is now a postdoctoral fellow at Le Laboratoire des Sciences du Climat et de l & # 39; Environnement in France, explains. “Our results mean that the current models overestimate the carbon losses in the Amazon rainforest due to climate change. In this particular region, these forests may actually be able to maintain or even increase the rates of photosynthesis if they are warmed and dried a little in the future. "
Gentine and Green note, however, that this sensitivity was only determined using existing data. Should the drought increase to levels that are not currently observed, this could actually change. Indeed, the researchers found a turning point for the most severe drought stress episodes, during which the forest could not maintain its level of photosynthesis. So, say Gentine and Green, "our results are certainly not an excuse not to reduce our carbon emissions."
Gentine and Green continue to work on issues related to vegetation water stress in the tropics. Green is currently focused on developing a water stress indicator using remote sensing data (a set of data that can be used to determine when a forest is under stress), quantifying the effects of water stress on plant carbon uptake, and mapping it to ecosystem characteristics.
"So much scientific research is being published today that our current ecosystems may not survive in the face of climate change, potentially accelerating global warming through feedback," added Green. "It was nice to see that some of our estimates of the approximation to mortality in the Amazon rainforest may not be quite as bad as we previously thought."
About the study
The study is titled "Photosynthesis of the Amazon rainforest is increasing in response to the drought in the atmosphere."
Authors are: J. K. Green 1,2; J. Berry 3; P. Ciais 2; Y. Zhang 1.4; P. Gentine 1.5
1 Department of Earth and Environmental Engineering, Columbia Engineering
2Laboratoire des Sciences du Climat et de l & # 39; Environnement (LSCE), Gif sur Yvette, France
3 Carnegie Institution for Science, Stanford, CA.
4 Department of Earth and Environmental Sciences, Lawrence Berkeley National Laboratory
5The Earth Institute at Columbia University
The study was supported by the NASA Earth and Space Science Fellowship (NNX16AO16).
The authors state that they have no competing interests.
DOI: 10.1126 / sciadv.abb7232
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